In coherent optical communication systems, optical amplifiers are often used at the output of the transmitter (Tx). Additionally, in power- and cost-constrained systems, a single laser may be shared between the transmitter and local oscillator (LO) in the receiver (Rx). In such a system using a Tx optical amplifier, the output Tx optical signal to noise ratio (OSNR) is typically limited by amplified spontaneous emission (ASE) noise due to the optical amplifier. The OSNR can be improved by reducing the insertion loss of the transmitter or by increasing the laser power dedicated to the transmitter. Typically, both of these parameters are fixed.
FIG. 1 is a schematic block diagram 100 of a prior art coherent optical transceiver 102, in which in-line power monitor photodiodes and fiber optic splices are omitted. A WDM coupler couples the light from pump laser 114 into the Tx EDFA 106. The Tx photodiode (PD) 116 is used to control the Tx EDFA 106 gain via a feedback loop 118 to control the pump laser 114 current. The light from the tunable laser 108 is shared between Tx 110 and Rx 112 and its splitting ratio is dictated by the required LO power PLO as explained below. The incoming receiver signal power Pr and the target photo current Ir ∝√{square root over (PLoPr)} determine the LO power PLO and, therefore, the Tx power PTx. As shown in FIG. 1, an electrical TX input 120, an optical Tx output 122, an optical RX input 130 and an electrical Rx output 132 are present.
The proportion of the system laser power dedicated to the Tx, PTx, is dictated by what is needed for the LO in the receiver. The required LO power, PLO, is determined by the amount of gain one needs in the receiver. In coherent intradyne detection, the input received optical signal, Pr, is mixed with a higher powered local oscillator PLO near the same frequency in order to boost the power in the mixing products at the intermediate frequencies ωIF=ωr−ωLO, where ωr is a frequency within the signal bandwidth. Given a signal input power Pr, detector responsivity R, and target current output Ir∝R√{square root over (PLO)}√{square root over (Pr)}, the LO power is fixed.
There is a need for systems that provide an optical communication system with improved optical signal to noise ratio.